It is often necessary or desired to provide a coating of a particular substrate. For example, in the video electronics industry it is often desired to coat panels which will serve as flat panel displays (FPD) to be incorporated into television sets, computer monitors and the like. It is important in such applications to ensure the accuracy and consistency of coating thicknesses across the panel.
A commonly employed method of coating flat panel displays is to have a stationary head extruding fluid at a particular rate over linearly moving panels however, moving heads are also used. Using such a configuration, the coating consistency is affected by a number of parameters such as the gap between the head and the panel surface, the variation in this gap as the panel moves, the dimensional consistency of the panel, the mechanical tolerances of the slot in the dispensing head, the pump characteristics and the fluid flow in the gap between the slot and the panel.
In prior art systems, variation in the height of the dispenser or extrusion head with respect to the panel can cause breaking of the coating bead and/or variation in coating thickness. The causes of such height variation include part dimension variation, part placement error, and gradual drift in machine dimensions over time. Accordingly, there is a need in the art for a system and method for ensuring constant height or "head gap" of a dispenser or extrusion head over a substrate being coated.
An independent contributor to possible variation in the height of the dispenser or dispensing head with respect to the substrate stems in fact from variation in machine dimensions rather than just variations in part placement and part dimensions. This variation in machine dimensions can result from a slow drift in mechanical dimensions over time, such as from the gradual bending of metal parts, or wearing of certain surfaces. The variation may also arise from thermal cycling causing expansion and contraction, and "settling" of surfaces/bearings under gravity.
Yet another possible contributor to possible variation in the height of a dispenser over the substrate to be coated could be variation in vertical position of the chuck holding the substrate along the direction of relative travel between the dispenser and the substrate.
Two separate problems should be distinguished here. There is the initial problem of establishing an accurate head gap when preparing for a coating operation. It is one problem in the art that variation in this initial head gap can exist between different coating runs. Separately, once this head gap has been initially established, there is the problem of accurately maintaining this head gap throughout a single coating process.
Prior art methods for solving the stated problems have involved manually inserting shims of varying thicknesses to set the head gap. This approach is deficient for several reasons. Although a head might be accurately established when the shims are first inserted, gradual drift in the dimensions of machine parts and the connections between them can cause the head gap to gradually change. It is important to note here that head gap changes of a few microns can adversely affect the coating bead.
A further problem with the prior art shim method is that even if the head is initially established with sufficient accuracy, the shims cannot compensate for subsequently introduced head gap variation arising from dimensional variation in the substrate or the chuck, part misplacement, or imperfect leveling of the substrate or supporting equipment. Therefore the manually inserted shims do not address the issue of maintaining the head gap throughout a coating process.
Therefore there is a need in the art for a system and method for accurately establishing the initial head gap in a coating apparatus prior to beginning a coating operation.
There is a further need in the art for a system and method for establishing the head gap which is independent of gradual change in the dimension and position of parts of the coating apparatus over time.
There is a still further need in the art for a system and method for accurately maintaining a properly established initial head gap throughout the performance of a coating operation.
There is a still further need in the art for a system and method for accurately maintaining a head gap throughout a coating operation independent of variation in dimensions of the part to be coated, error in part placement, and imperfect leveling of the structure supporting the part to be coated.